Plastisols are used widely in auto body construction. The main applications are sealers between panels, bonding of sheets in body and paint shops, and adhesives. They are also used for protection of external areas, such as for undercoating and rocker panel coating.
For this purpose, plastisols based on PVC polymers or PVC copolymers, e.g., as described in DE 31 11 815, are most often used. Such plastisols consist of fine polymer particles dispersed in a nonvolatile plasticizer. At room temperatures, the solid particles are not soluble in the liquid phase, at higher temperatures (the gel temperature), the polymer particles dissolve in the plasticizer. On cooling, the homogeneous solution solidifies to an elastic or rigid film. Normally the plastisols contain additives such as fillers, rheological additives, stabilizers, pigments, solvents, water-absorbing substances, adhesion promoters, etc. The adhesion promoters facilitate adhesion to the surfaces of the respective substrates. Examples of such substrate surfaces are oily steel, galvanized or tinplated steel, electro-coated sheets, aluminum, etc.
Suitable adhesion promoters for sealants or coatings include polyaminoamides, polyamines, epoxy resins, reaction products of polyamines and epoxy resins, blocked isocyanates, organofunctional silanes, mixtures of urotropine and resorcinol, and combinations thereof.
In automotive construction, plastisols or other sealants and coating compounds can be applied to the oily substrates and subsequently degreased, phosphatized, and provided with an electro-dip coating which is baked at temperatures above 150.degree. C. Usually the compounds are applied to surfaces already primed cataphoretically; the jelling or fusing of the plastisols occurs with the baking of the subsequently applied base coats or top coats.
It is often noticed that white, light, or pastel colored finish coats of the vehicle become discolored (yellowish or brownish) at those points where the finish paints are applied on top of the sealing compounds or underbody coatings. Where the finish paints are applied directly on the electrocoat, they yellow much less. Such undesirable discolorations occur regardless of whether the sealing compound was jelled before overpainting or whether the unjelled sealing compound was provided with the finish coat by the wet-on-wet process and baked only thereafter. The yellowish discoloration appears after a period of time, sometimes after only weeks or months. High moisture and elevated temperature accelerate the formation of the discolorations.
The observed discolorations are due, inter alia, to amines which migrate to the surface of the finish coats in small amounts and become transformed by oxidation into colored substances at the surface. These amines may originate from the electrophoretic coating or from the additives. The plasticizers in the sealants assist in the migration of these discoloring substances to the surface. The coats applied over the sealants absorb the plasticizers, leading to flexibilization of the paint and facilitating the diffusion of the dissolved substances. Many of the lacquer systems used today are acid-curing; that is, their crosslinking takes place under the influence of acid catalysts. Sealants and coatings preferably contain chalks or other basic fillers which partially neutralize these acid catalysts. The reduced concentration of the catalysts over the sealants leads to coatings which are less crosslinked. Thus the low crosslinking density also encourages the diffusion of the discoloring substances from the underlying plasticizer-containing materials to the surface.
It has been proposed to bind the unwanted amines by addition of acid substances to the sealant. In DE-PS 38 21 926, acid ion exchangers in powder form are added to the sealant. In DE-OS 38 43 994, the potentially color-giving amines are bound by reaction with tannins which are added to the plastisol. When acid substances are added, however, a part of the adhesion promoters may be absorbed; thus, the adhesion promoters are no longer available for interactions with the surfaces; for example, for the reaction with the electrocoated sheets or with the galvanized steel surfaces.
Finely divided acid ion exchangers are disadvantageous not only for reasons of cost, but also because the fine dust of such powders is extremely irritating to the respiratory tract and may lead to allergic reactions.